1. Field of the Invention
The instant disclosure relates to a buck converter; in particular, to a buck converter and control method therefor.
2. Description of Related Art
Please refer to FIG. 1 in conjunction with FIG. 2A and FIG. 2B, FIG. 1 shows a circuit diagram of a conventional buck converter. The conventional buck converter 1 comprises a high-side N-type MOSFET 11, an inductor L, a capacitor C, a low-side N-type MOSFET 12, a control circuit 13, a boot-strap capacitor Cboot and a charge controller 14. An inductor L and a capacitor C of the buck converter 1 constitute an output filter for providing an output voltage VOUT. The control circuit 13 comprises a gate driving circuit 131, a feedback comparator 132 and a feedback circuit constituted of resistors R1, R2 (which dividing the output voltage VOUT to generate a feedback signal FB. The gate driving circuit 131 comprises a gate driving logic 1311, a buffer 1312 and a buffer 1313. The feedback comparator 132 compares the feedback FB and the reference voltage VREF. In the buck converter 1, the gate of the high-side N-type MOSFET switch 11 is turned-on or turned-off by the voltage of the boot-strap capacitor Cboot, wherein the voltage of the boot-strap capacitor is charged by the input voltage VIN through the charge controller 14 when the low-side N-type MOSFET 12 is turned-on. When the buck converter is in a light load operation, the low-side N-type MOSFET would not be turned-on most of the time. Ideally, the voltages of a first terminal BST and a second terminal SW of the boot-strap capacitor Cboot are shown in FIG. 2A. However, in practical, the voltage level of the boot-strap capacitor Cboot would decrease gradually due to leakage current, as shown in FIG. 2B. Therefore, when the output voltage VOUT is not enough, the voltage of the gate driving signal (varying with the voltage of the capacitor Cboot) controlling the high-side N-type MOSFET would be not high enough. Then the resistance of the high-side N-type MOSFET becomes large, and the high-side N-type MOSFET may be damaged due to large power dissipation leading to burnout of circuit.
Please refer to FIG. 1 in conjunction with FIG. 2C, FIG. 2C shows a curve diagram of the voltages at two terminals of a boot-strap capacitor of a conventional buck converter while the boot-strap capacitor being charged when the voltages across the two terminals of the boot-trap capacitor are less than a threshold voltage. U.S. Pat. No. 5,627,460 illustrates a technique avoiding the in-sufficient gate driving voltage of the high-side N-type MOSFET due to low voltage of the boot-strap capacitor Cboot. This technical solution turns on the low-side N-type MOSFET when the BST terminal voltage is below Vt, thus keeps the Cboot being re-charged frequently, as shown in FIG. 2C. However, it may cause efficiency reduction of the buck converter.